"Methanation" is a catalytic reaction which yields methane gas from carbon monoxide, carbon dioxide or mixtures thereof and hydrogen according to the equations: EQU CO+3H.sub.2 =CH.sub.4 +H.sub.2 O, .DELTA.H=-52.7 Cal (1) EQU CO.sub.2 +4H.sub.2 =CH.sub.4 +2H.sub.2 O, .DELTA.H=-43.6 Cal (2)
The limited availability of methane from natural sources coupled with the enormous utility of methane as a clean, sulfur free fuel have combined to create a great need for "synthetic natural gas". Methane made by methanation holds great economic significance because the reactants can be obtained by a reaction involving readily available coal with steam according to the basic equations: EQU 2C+2H.sub.2 O=2CO+2H.sub.2 ( 3)
and EQU CO+H.sub.2 O=CO.sub.2 +H.sub.2 ( 4)
to produce EQU 2C+3H.sub.2 O=CO.sub.2 +CO+3H.sub.2 ( 5)
Reactions (1) and (2) are highly exothermic and are reversible so that high temperatures tend to reduce the yield of methane. Accordingly, heat removal poses a significant problem in all methanation processes. In addition, many of the processes either do not convert carbon dioxide to methane or are sensitive to the presence of sulfur compounds and/or an excessive amount of water in the process gases.
Conventional prior art methanation processes are conducted by usually passing the gaseous reactants through a packed or fluidized bed of a catalyst which is typically nickel or a nickel alloy with platinum. Such a process is disclosed, for example, in U.S. Pat. No. 3,930,812 issued to Harris et al. However, packed bed processes such as that of Harris et al are characterized by temperature control problems and a large pressure drop across the reactor. Dorschner et al, in U.S. Pat. No. 2,662,911, conduct the reaction in a plurality of catalyst packed tubes vertically arranged in a water-containing drum. Dorschner, in U.S. Pat. No. 2,740,803, also discloses methanation in a fluidized bed provided with double-wall bayonette type heat exchangers. This latter Dorschner patent also discloses an embodiment wherein the catalyst is contained in "contact tubes, vertically arranged in a water-containing drum having diameters which progressively decrease from the top to the bottom". These methods, like the more conventional packed bed methods, are also characterized by high pressure drops across the reactor.
Further, in most, if not all, of the foregoing prior art methanation processes characterized by the use of granular or particulate catalysts, there is a tendancy to form coke on their surfaces and plug up over prolonged periods of time.
Lastly, it is known to use Raney nickel as a catalyst in methanation processes. See, for example, "Methanation Studies on Nickel-Aluminum Flame Sprayed Catalysts" by Baird and Steffgen, Journal of Industrial Engineering Chemistry, Product Research Development, Volume 16, No. (1977), in which the use of a methanation catalyst prepared by flame spraying aluminum onto a nickel surface followed by heating to form a Raney-type alloy and then activating it with a caustic leach is discussed. In this article, it was found that there was a strong correlation between the NiAl.sub.3 (beta nickel) content in the unleached alloy and the methanation activity of the leached catalyst. No mention of the use of molybdenum, titanium, tantalum or ruthenium as alloying ingredients of the nickel is given or suggested.
Additional studies involving nickel-molybdenum methanation catalysts were reported by Wilhelm, Tsigdinos and Fernece, "Preparation and Activity of Nickel-Molybdenum Methanation Catalysts"; Chemical Uses of Molybdenum Proceedings, 3rd International Conference (1979). However, no mention of Raney treatment is given or suggested. When these catalysts were used even at elevated temperatures and pressures, useful CO conversions were reported to be in the neighborhood of only 80 to 90 percent. No suggestion of applicability to CO.sub.2 is given.
Most recently, U.S. Pat. No. 4,043,946 issued to Sanker et al discloses a method for making a supported Raney nickel catalyst containing up to 5 percent molybdenum which, when tested for methanation activity, was found to require a temperature on the order of 320.degree. C. to achieve a CO conversion of about 99 percent. No mention is made of potential applicability to CO.sub.2.